To minimize the web loss by changing the method of predicting the number according to the presence/absence of coil changes and the difference (the width or the length) in changing the size of PS(presensitized) plate.

When the coil 14 is changed, the number of the passed webs out of the webs 12 present between a first detection sensor 20 and a sheet cutter 32 is predicted from the distance from the first detection sensor 20 to the sheet cutter 32, the designated length of the PS plate, the designated length of the PS plate, and the distribution of defective portions detected by the first detection sensor 20. When the width of the PS plate is changed without changing the coil, the number of the passed webs before changing the width out of the webs present between a second detection sensor 34 and the sheet cutter 32 is predicted at the position of the second detection sensor from the distance from the second detection sensor 34 and the sheet cutter 32 and the feed of the defective portion from the second detection sensor 34. When the length of the PS plate is changed, the number of the passed webs before changing the length is counted at the position of the sheet cutter 32 from the feed of the defective portion from the second detection sensor.